FIG. 1 illustrates a cross-sectional view of a human eye 10 having an anterior chamber 12 and a posterior chamber 14 separated by an iris 30. Within the posterior chamber 14 is a capsular bag 16 which holds the eye's natural crystalline lens 17. Light enters the eye by passing through cornea 18. The cornea and crystalline lens act together to direct and focus the light onto a retina 20. The retina is connected to optic nerve 22 which transmits images received by the retina to the brain for interpretation. Eye 10 has a visual axis VA.
In response to the sharpness of the image received by the retina, the brain operates to contract or relax ciliary muscle 26. Ciliary muscle 26 is disposed within ciliary body 28, and upon contraction of the ciliary muscle, the ciliary body is caused to move. To achieve near focus accommodation, the ciliary muscle is contracted thereby causing the ciliary body to relax tension on zonules 27 which permits the capsular bag and lens 17 to become more rounded. To achieve far focus (i.e., disaccommodation), the ciliary muscle is relaxed thereby increasing tension on zonules 27 which causes the capsular bag and lens 17 to become flatter.
In an eye where the natural crystalline lens has been damaged (e.g., clouded by cataracts), the natural lens is no longer able to properly focus and/or direct incoming light to the retina. As a result images become blurred. A well known surgical technique to remedy this situation involves removal of a damaged crystalline lens through a hole in the capsular bag known as a capsularhexis (also referred to simply as a rhexis). Subsequently, an artificial lens known as an intraocular lens (IOL) can be placed into the evacuated capsular bag through the rhexis.
Conventional IOLs are typically fixed-focus lenses. Such lenses are usually selected to have a power such that the patient has a fixed focus for distance vision, and the patient requires spectacles or contact lenses to permit near vision. In recent years extensive research has been carried out to develop IOLs having variable focus capability. Such IOLs are known as accommodating IOLs (AIOLS). The term “AIOLs” refers to both single-element and multi-element optical systems.
AIOLs permit a wearer to have accommodative vision. AIOLs are typically located in the posterior chamber (e.g., in the capsular bag) and provide variable focal power in accordance with tension or a lack of tension exerted on the capsular bag 16 as a result of contraction and relaxation of the ciliary muscle. FIG. 2 shows a cross section of an example of a two-element IOL 24 in capsular bag 16. IOL 24 comprises an anterior lens element 42 and a posterior lens element 44 that are connected to one another by bias elements 46. The bias elements permit lens elements 42 and 44 to translate relative to one another to achieve accommodation and disaccommodation. Further details of IOL 24 are given in U.S. Pat. No. 6,488,708 issued Dec. 3, 2002, to Sarfarazi
Designs of AIOLs are commonly made using eye models that are based on assumptions about the mechanical properties of the capsular bag and how the AIOL will interact with the capsular bag to achieve a given amount of translation. Knowledge regarding the mechanical properties of the capsular bag and interaction with the AIOLs is evolving as clinical results from AIOL implantations are becoming available, and as computer modeling of the eye has advanced.